/*
 * Copyright (c) 2010 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.api.client.escape;

/**
 * An object that converts literal text into a format safe for inclusion in a
 * particular context (such as an XML document). Typically (but not always), the
 * inverse process of "unescaping" the text is performed automatically by the
 * relevant parser.
 * 
 * <p>
 * For example, an XML escaper would convert the literal string {@code
 * "Foo<Bar>"} into {@code "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} from
 * being confused with an XML tag. When the resulting XML document is parsed,
 * the parser API will return this text as the original literal string {@code
 * "Foo<Bar>"}.
 * 
 * <p>
 * An {@code Escaper} instance is required to be stateless, and safe when used
 * concurrently by multiple threads.
 * 
 * <p>
 * Several popular escapers are defined as constants in the class
 * {@link CharEscapers}.
 */
public abstract class Escaper {
  /**
   * Returns the escaped form of a given literal string.
   * 
   * <p>
   * Note that this method may treat input characters differently depending on
   * the specific escaper implementation.
   * <ul>
   * <li>{@link UnicodeEscaper} handles <a
   * href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> correctly, including
   * surrogate character pairs. If the input is badly formed the escaper should
   * throw {@link IllegalArgumentException}.
   * </ul>
   * 
   * @param string the literal string to be escaped
   * @return the escaped form of {@code string}
   * @throws NullPointerException if {@code string} is null
   * @throws IllegalArgumentException if {@code string} contains badly formed
   *         UTF-16 or cannot be escaped for any other reason
   */
  public abstract String escape(String string);
}
